Author Topic: BMW F900 XR 16/44 front Sprocket  (Read 285 times)

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Offline dlitchmd@citlink.net

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BMW F900 XR 16/44 front Sprocket
on: September 24, 2021, 04:09:00 PM
After having spent a weekend in south eastern Kentucky riding roads in the coal mining country at speeds 20 to 50 mph, I was constantly changing gears trying to avoid lugging the engine or revving it to annoying vibrations.  Sweet Spot on this engine seems  between 3000 and 4000 RPM.

So I decided to change the front sprocket from 17 stock to a 16 tooth (SprocketCenter with rubber dampening) giving a 5.85% change in rpm’s and a tremendous boost in low end and actually overall performance where most of my riding occurs.  I avoid interstates when possible.  This resulted in a far more pleasant bike for me.  Change in speedometer was only 1 mph faster than actual GPS.

Torque values taken from another forum posting are:
50nm Counter sprocket bolt
38nm for larger bolts on foot plate
28 nm for smaller bolts
200 nm for a big swing arm bolt

Start bolts and aline plate before tightening.  200nm is 147 ft lbs and required a breaker bar for me.

I was able to pull the front sprocket off with chain attached without loosening the chain but that would probably make it easier.  Putting on the new smaller sprocket was easy.

This required a minimal amount of original chain adjustment.  See owner’s manual.

Of course all of this is my opinion and you may find things differently. Do so at your own risk.

Online rcb78

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Re: BMW F900 XR 16/44 front Sprocket
Reply #1 on: September 24, 2021, 05:41:49 PM
So I just did something very similar except I went to a 46t rear sprocket and simply replaced the 17t with a rubber damped version.  My motive was simple experimentation.  Something to consider about dropping to a 16t front is that you 'may' see some odd wear related to that choice, the teeth of the 16t will see the same tooth/chain link contact point every 8 chain rotations vs 17 for stock or the combo I chose.  Additionally both front and rear will see the same spot of the chain every 88 rotations vs 374 (stock) or 391 (17/46).  Play with www.gearingcommander.com a little bit, it's a cool site that lets you play with all those numbers.  There's no guarantee that it'll play out as accelerated wear, but the potential is there.
The other thing I didn't like about the 16t was how much more room it gave the chain to jump if it went slack enough, the 17t keeps it tight to the guard.  Again, this is only a worst case scenario but still worth considering since jumping a chain up front can do some serious damage.
I need to count the links on the chain to see what I ended up with, sprocket center sends what they think you need if you change ratios when ordering a kit.  I had to pull a link out of what they sent, but the wheel is still further back in the swing arm that it was with the smaller gear, so one link longer than stock maybe??
My first impressions is that the rubber damped front cog pulled eliminated the power pulses you get at lower rpms when rolling on, i.e. 2500-3500 rpms, so the bike feels smoother when cruising along at lower speeds on surface streets.  80mph is just barely over 5k which is just about perfect for my freeway commute.

Offline dlitchmd@citlink.net

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Re: BMW F900 XR 16/44 front Sprocket
Reply #2 on: September 24, 2021, 08:17:35 PM
I think we both agree that the drive ratios needed to be changed. And I don’t disagree with you about how to go about it. I too wanted to work with the back sprocket but I wanted a little more percent change than two extra teeth added the rear sprocket provided. It seems no one makes a 47 tooth rear sprocket. Also it was going to be more expensive changing out the rear sprocket and since I only had 1000 miles on the new bike, I felt there was minimal miss matching and it was an easier overall change especially since I was unsure if the stock chain was going to be long enough with the larger sprocket.  Good point about the chain jumping  the smaller sprocket but I am pretty anal about tire pressures, oil levels and chain adjustments. 

Did you notice a significant improvement in your bikes  performance and were you also pleased? 

Online rcb78

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Re: BMW F900 XR 16/44 front Sprocket
Reply #3 on: September 24, 2021, 08:54:46 PM
Only reason I was doing it at all was because I was due for a new chain, red dust of death and it measured beyond spec.  I totally agree about wanting the 47t rear, THAT is what I really wanted but like you said they are nowhere to be found, at least not in steel.  Not significant, but certainly noticeable.  The speeds I cruise out, the shorter gearing helps me be in a slightly better place in the powerband for overtaking.